The present invention relates to a method for forming radiation images.
As is well-known in the art, an organic compound having a photosensitivity is an indispensable material in the field of production of semi-conductor devices. This material is ordinarily used as a protective layer when a silicon oxide layer as a mask for impurity diffusion is etched. More specifically, in the process for production of semi-conductor devices, diffusion of impurities is often accomplished by forming holes of a specific pattern or masking windows on a silicon oxide layer formed on a semiconductor substrate and diffusing impurities into the substrate through these holes. Accordingly, the precision of diffusion of impurities into the semiconductor substrate depends greatly on the precision of formation of masking windows, namely the masking precision and mask aligning precision of a photosensitive material.
As materials that can be used as the protective layer, there have heretofore been used KPR and KTFR (trademarks for photosensitive materials manufactured by Eastman Kodak Co.) and AZ 1350 (trademark for a photosensitive material manufactured by Shipley Co.). Such photosensitive materials are coated on a silicon oxide layer and exposed to ultraviolet rays imagewise through a suitable mask, development is carried out by using a liquid developer and the silicon oxide layer is chemically etched by using the resulting imagewise photosensitive material layer as a protective layer.
This method is a modification of the photolithographical method heretofore practically utilized for photographic production of printing plates. However, according to this method including exposure to ultraviolet rays, it is difficult to reduce the etching size and precision below the critical wave length of rays used for the exposure (about 0.4 .mu.m). Recently, however, large scale integrated circuits (LSI circuits) comprising great numbers of elements and connections built in one small semi-conductor substrate are eagerly desired and vigorously developed, and in order to improve the packing density in such circuits, development of a lithographic technique capable of very fine and precise etching is now desired in the art. As means for meeting such desire, there have been studied photolithographic methods using electron beams or X-rays instead of ultraviolet rays.
Since the wave length of electron beams is much shorter than the wave length of ultraviolet rays as is well-known in the art, they are suitable for very fine etching. Further, since control of intensity modulation or deflection is relatively easy in case of electron beams, there is attained an advantage that it is possible to control precisely the irradiation position and dose. By utilizing this advantage, it is made possible to further minimize semi-conductor elements and improve the frequency characteristics and packing density in the semi-conductor elements. Also the wave length of X-rays is much shorter than the wave length of ultraviolet rays, and therefore, X-rays can be used for precise and fine etching as well as electron beams.
When electron beams or X-rays are used instead of ultraviolet rays in the photolithography, a radiation-sensitive material must be used as a protective layer-forming material corresponding to the photosensitive material.
One of important properties required of a radiation-sensitive material to be used as the masking material in the lithographic process using radiations is a high sensitivity to radiations. If a polymeric compound having a high sensitivity to radiations is used, the dose of radiations applied to the material can be greatly reduced and the time of exposure can be shortened to improve the operation efficiency.
Another important property required of a radiation-sensitive material to be used as the masking material in the lithographic process is a high contrast characteristic. By the term "high contrast characteristic" is meant such a property that the thickness of the layer of the developed material is readily changed depending on the change of the dose. If a radiation-sensitive polymeric compound having such high contrast characteristic is used, the boundary can be made remarkably definite between the material-remaining part and the material-free part on the surface of the substrate. Accordingly, a protective layer having a clear pattern of a high resolving power can easily be formed and the fineness of etching can be remarkably improved.
As the polymeric compound having a very high sensitivity to radiations, there are known compounds having in the molecule a number of epoxy linkages ##STR1## such as compounds disclosed in Japanese Patent Publication No. 7495/74. Such polymeric compound, for example, epoxidized 1,4-polybutadiene, is prepared by epoxidizing carbon-to-carbon double bonds (.dbd.C.dbd.C.dbd.) of 1,4-polybutadiene which is a synthetic rubber with an organic per acid such as peracetic acid. The so prepared epoxy structure-containing compound has a very high radiation sensitivity, but its contrast characteristic is not good.